var alph = new Array();
var word = WScript.StdIn.ReadLine();

for (i = 0; i < word.length; i++)
{
    var letter = word.charAt(i);

    if (alph[letter] == undefined)
    {
        alph[letter] = 0;
        alph[letter]++;
    }
    else
        alph[letter]++;
}
var entropy = 0;

for (var i in alph)
{
    var pi = alph[i] / word.length;
    entropy += pi * (Math.log(pi) / Math.log(alph[letter])); // log(pi) по основанию alph[letter]
}
WSH.echo(-1 * entropy.toFixed(5));